Combat in Chainmail is simply rolling two six-sided dice, and you either defeated the monster and killed it…or it killed you. It didn’t take too long for players to get attached to their characters, and they wanted something detailed which Chainmail didn’t have. The initial Chainmail rules was a matrix. That was okay for a few different kinds of units, but by the second weekend we already had 20 or 30 different monsters, and the matrix was starting to fill up the loft.
I adopted the rules I’d done earlier for a Civil War game called Ironclads that had hit points and armor class. It meant that players had a chance to live longer and do more. They didn’t care that they had hit points to keep track of because they were just keeping track of little detailed records for their character and not trying to do it for an entire army. They didn’t care if they could kill a monster in one blow, but they didn’t want the monster to kill them in one blow.